A helicopter drops water on to a forest fire near Pitlochry, Scotland, Britain, may 4, 2017. REUTERS/Russell Cheyne 

A helicopter drops water on to a forest fire near Pitlochry, Scotland, Britain, may 4, 2017. REUTERS/Russell Cheyne 

A police helicopter flies over a burned bus damaged during violent clashes between rival gangs at Brasil avenue near Cidade Alta slum in Rio de Janeiro, Brazil, May 2, 2017. REUTERS/Ricardo Moraes 

A helicopter flies over a Union Flag at half-mast in Westminster the day after an attack, in London, Britain March 23, 2017. REUTERS/Stefan Wermuth 

Wreckage of a helicopter is seen after it crashed in Istanbul, Turkey, March 10, 2017. REUTERS/Osman Orsal 

Wreckage of a helicopter is seen after it crashed in Istanbul, Turkey, March 10, 2017. REUTERS/Osman Orsal TPX IMAGES OF THE DAY 

Crews work on electrical power lines after an evacuation order was lifted for communities downstream from the Lake Oroville Dam in Oroville, California, U.S. February 15, 2017. REUTERS/Jim Urquhart 

Helicopters carry rocks to the Lake Oroville Dam after an evacuation order was lifted for communities downstream in Oroville, California, U.S. February 15, 2017. REUTERS/Jim Urquhart 

Helicopters carry rocks as water is released from the Lake Oroville Dam after an evacuation was ordered for communities downstream from the dam in Oroville, California, U.S., February 14, 2017. REUTERS/Jim Urquhart 

Helicopters carry rocks as water is released from the Lake Oroville Dam after an evacuation was ordered for communities downstream from the dam in Oroville, California, U.S., February 14, 2017. REUTERS/Jim Urquhart TPX IMAGES OF THE DAY 

People watch as helicopters haul rock to the Lake Oroville Dam after an evacuation was ordered for communities downstream from the dam in Oroville, California, U.S. February 13, 2017. REUTERS/Jim Urquhart 

A wild horse is herded into corrals by a helicopter during a Bureau of Land Management round-up outside Milford, Utah, U.S., January 7, 2017. REUTERS/Jim Urquhart SEARCH "WILD HORSE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. TPX IMAGES OF THE DAY 

French rescue services work at the scene of an accident involving fifty vehicles which crashed due to fog on the road between La Roche-sur-Yon and Sables-d'Olonne, France, that killed five and injured more than forty, December 20, 2016. REUTERS/Stephane Mahe 

A French Navy helicopter sits on the deck elevator aboard the Charles-de-Gaulle aircraft carrier operating in the eastern Mediterranean Sea on December 9, 2016. REUTERS/Stephane De Sakutin/Pool

A French Navy helicopter is moved from the deck elevator of the Charles-de-Gaulle aircraft carrier operating in the eastern Mediterranean Sea on December 9, 2016. REUTERS/Stephane De Sakutin/Pool

French President Francois Hollande (C) arrives on board the Charles-de-Gaulle aircraft carrier operating in the eastern Mediterranean Sea on December 9, 2016. REUTERS/Stephane De Sakutin/Pool

The helicopter carrying French President Francois Hollande prepares to land on the flight deck of the Charles-de-Gaulle aircraft carrier operating in the eastern Mediterranean Sea on December 9, 2016. REUTERS/Stephane De Sakutin/Pool

French President Francois Hollande (2ndL) walks on the flight deck as he arrives on board the Charles-de-Gaulle aircraft carrier operating in the eastern Mediterranean Sea on December 9, 2016. REUTERS/Stephane De Sakutin/Pool

French President Francois Hollande arrives on board the Charles-de-Gaulle aircraft carrier operating in the eastern Mediterranean Sea on December 9, 2016. REUTERS/Stephane De Sakutin/Pool

European Union foreign policy chief Federica Mogherini (C) talks to reporters during the Black Blade military exercise involving several European Union countries and organised by the European Defence Agency, at Florennes airbase, Belgium November 30, 2016. REUTERS/Yves Herman TPX IMAGES OF THE DAY 

The Marine One helicopter with U.S. President Barack Obama on boards lands on the South Lawn of the White House in Washington, U.S., November 29, 2016. REUTERS/Yuri Gripas 

A general view of Vattenfall's offshore accommodation platform, 90 kilometres west of Esbjerg, Denmark, September 21, 2016. Picture taken September 21, 2016. To match EUROPE-OFFSHORE/WINDPOWER REUTERS/Nikolaj Skydsgaard 

Evacuees formerly stranded in the earthquake-affected town of Kaikoura stand alongside the helicopter that brought them to the town of Woodend, near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

Evacuees formerly stranded in the earthquake-affected town of Kaikoura walk away from the New Zealand Air Force helicopter that brought them to the town of Woodend, near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

Evacuees formerly stranded in the earthquake-affected town of Kaikoura stand alongside the helicopter that brought them to the town of Woodend, near Christchurch, New Zealand November 16, 2016. REUTERS/Lincoln Feast 

Evacuees formerly stranded in the earthquake-affected town of Kaikoura stand alongside the helicopter that brought them to the town of Woodend, near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

Evacuees formerly stranded in the earthquake-affected town of Kaikoura stand alongside the helicopter that brought them to the town of Woodend, near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

A helicopter brings evacuees formerly stranded in the earthquake-affected town of Kaikoura to the town of Woodend, near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast TPX IMAGES OF THE DAY

Helicopters bringing evacuees formerly stranded in the earthquake-affected town of Kaikoura land in the town of Woodend near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

A helicopter brings evacuees formerly stranded in the earthquake-affected town of Kaikoura to the town of Woodend, near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

Evacuees formerly stranded in the earthquake-affected town of Kaikoura stand alongside the helicopter that brought them to the town of Woodend, near Christchurch, New Zealand November 16, 2016. REUTERS/Lincoln Feast 

An evacuee (L) formerly stranded in the earthquake-affected town of Kaikoura gives a thumbs up to the crew of a helicopter that brought them to the town of Woodend near Christchurch, New Zealand, November 16, 2016. REUTERS/Lincoln Feast 

An army helicopter flies over al Khazer refugee camp, Iraq November 11, 2016. REUTERS/Zohra Bensemra 

Serbian soldiers disembark from MI-8 helicopter during a joint Serbian-Russian military training exercise "Slavic Brotherhood" in the town of Kovin, near Belgrade, Serbia November 7, 2016. REUTERS/Marko Djurica 

Republican U.S. presidential nominee Donald Trump (center R) arrives by helicopter for a campaign rally in Naples, Florida, U.S. October 23, 2016. REUTERS/Jonathan Ernst 

Republican U.S. presidential nominee Donald Trump (R) arrives by helicopter for a campaign rally in Naples, Florida, U.S. October 23, 2016. REUTERS/Jonathan Ernst 

Republican U.S. presidential nominee Donald Trump departs in his helicopter after a campaign rally in Naples, Florida, U.S. October 23, 2016. REUTERS/Jonathan Ernst 

Republican U.S. presidential nominee Donald Trump waves through the window as he departs in his helicopter after a campaign rally in Naples, Florida, U.S. October 23, 2016. REUTERS/Jonathan Ernst 

Republican U.S. presidential nominee Donald Trump departs in his helicopter after a campaign rally in Naples, Florida, U.S. October 23, 2016. REUTERS/Jonathan Ernst 

A stiker is pictured on an helicopter at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

French Minister for Ecology, Sustainable Development and Energy Segolene Royal inaugurates the Volta, an all electric helicopter at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

The Volta, an all electric helicopter takes off at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

The Volta, an all electric helicopter takes off at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

French Minister for Ecology, Sustainable Development and Energy Segolene Royal inaugurates the Volta, an all electric helicopter at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

French Minister for Ecology, Sustainable Development and Energy Segolene Royal inaugurates the Volta, an all electric helicopter at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

French Minister for Ecology, Sustainable Development and Energy Segolene Royal inaugurates the Volta, an all electric helicopter at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

The Volta, an all electric helicopter takes off at the Paris Heliport in Issy-les-Moulineaux, France, October 19, 2016. REUTERS/Regis Duvignau 

A rescue helicopter takes part in a large-scale earthquake simulation exercise in Seoul, South Korea October 19, 2016. REUTERS/Kim Hong-Ji 

U.S. President Barack Obama walks from Marine One as he returns to the White House in Washington, U.S., October 14, 2016. REUTERS/Joshua Roberts 

U.S. President Barack Obama walks from Marine One as he returns to the White House in Washington, U.S., October 14, 2016. REUTERS/Joshua Roberts 

U.S. President Barack Obama walks from Marine One as he returns to the White House in Washington, U.S., October 14, 2016. REUTERS/Joshua Roberts 

U.S. President Barack Obama walks from Marine One as he returns to the White House in Washington, U.S., October 14, 2016. REUTERS/Joshua Roberts 

U.S. President Barack Obama arrives to board Air Force One for travel to Pennsylvania and Ohio from at the Joint Base Andrews, Maryland, U.S. October 13, 2016. REUTERS/Carlos Barria 

US forces deliver food supplies after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares 

US forces deliver food supplies after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares

US forces deliver food supplies after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares 

US forces deliver food supplies after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares 

Haitian run away as a helicopter of US forces takes off after leaving food suplies for residents after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares TPX IMAGES OF THE DAY

Haitians help US forces to unload a helicopter with food supplies after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares 

A Haitian man helps US forces to unload a helicopter carrying food supplies after Hurricane Matthew in Les Anglais, Haiti, October 12, 2016. REUTERS/Andres Martinez Casares